The present invention relates to a miniature movable device which is manufactured with a micro-machining technique such as photolithography or etching process, and in particular, to a miniature movable device including a movable part adapted to be displaced parallel to the sheet surface of a substrate and hinges which support the movable part in a displaceable manner.
A miniature movable device of the kind described above includes a miniature optical switch, a miniature optical attenuator, a miniature accelerometer and the like, and such a miniature movable device is generally manufactured using an SOI (Silicon On Insulator) substrate having a three layer structure including a single crystal silicon substrate on which another single crystal silicon layer is superimposed with an insulating layer interposed therebetween.
FIG. 1 shows the construction of an optical switch disclosed in U.S. Pat. No. 6,315,462, issued Nov. 13, 2001 (which is referred to hereafter as a literature 1), and the construction of the optical switch will be described below as an example of a conventional arrangement of a miniature movable device manufactured using such an SOI substrate.
Four fiber channels 112a to 112d are formed in a crisscross configuration in the top surface of a sheet-like composite substrate 111 which comprises an SOI substrate, and an area which is defined between the fiber channels 112a and 112b which are perpendicular to each other represents a drive formation 111′. A slot 113 is formed in the drive formation 111′ at an angle of 45° with respect to each of the fiber channels 112a and 112b, and a movable rod 114 is disposed in the slot 113.
A mirror 115 is mounted on one end of the movable rod 114, and is located at a center 116 of the fiber channels 112a to 112d which are disposed in the crisscross configuration. Support beams 117a and 117b have their one end connected to the opposite sides of the movable rod 114 at a location intermediate the length thereof, and the other ends of the support beams 117a and 117b are secured to fixing parts 119a and 119b through hinges 118a and 118b, respectively, which are in the form of leaf springs. Similarly, at the other end of the movable rod 114, support beams 117c and 117d have their one end connected to the opposite sides thereof, and the other ends of the support beams 117c and 117d are secured to fixing parts 119a and 119b through hinges 118c and 118d, respectively, which are also in the form of leaf springs, whereby the movable rod 114 is supported to be movable in the lengthwise direction thereof by the hinges 118a to 118d. 
The movable rod 114 is adapted to be driven by a comb-tooth type electrostatic actuator comprising movable comb tooth electrodes 121a to 121d which are fixedly mounted on the support beams 117a to 117d, respectively, and mating fixed comb tooth electrodes 122a to 122d which are fixedly mounted on the drive formation 111′.
When a voltage is applied across the movable comb tooth electrodes 121a, 121b and the fixed comb tooth electrodes 122a, 122b, an electrostatic force of attraction is developed to move the movable rod 114 in a direction toward the center 116. On the other hand, when a voltage is applied across the movable comb tooth electrodes 121c, 121d and the fixed comb tooth electrodes 122c, 122d, a resulting electrostatic force of attraction moves the movable rod 114 in a direction away from the center 116. In this manner, by driving the movable rod 114 with the comb tooth type electrostatic actuator, the mirror 115 can be inserted into or extracted from the center 116.
Optical fibers 123a to 123d are disposed in the four fiber channels 112a to 112d, respectively, and when the mirror 115 is inserted into the center 116, light which is emitted from the optical fiber 123a, for example, is reflected by the mirror 115 to impinge on the optical fiber 123d, and light which is emitted from the optical fiber 123b is reflected by the mirror 115 to impinge the optical fiber 123c. On the other hand, when the mirror 115 is extracted from the center 116, light emitted from the optical fiber 123a impinges on the optical fiber 123c, and light emitted from the optical fiber 123b impinges on the optical fiber 123d, and a switching of the optical path takes place in this manner.
The optical switch is manufactured by a process illustrated in FIGS. 2A to 2C. Specifically, as shown in FIG. 2A, an SOI substrate 130 including a single crystal silicon substrate 131 on which another single silicon crystal layer 133 is superimposed with an insulating layer 132 formed by an silicon oxide film interposed therebetween is provided. An intended mask 134 is formed on top of the single crystal silicon layer 133 by a patterning process. Portions of the single crystal silicon layer 133 which are exposed through the mask 134 is subject to a reactive ion etching (RIE) to remove the single crystal silicon layer 133 until the insulating layer 132 becomes exposed as shown in FIG. 2B.
A narrow width portion 135 of the single crystal silicon layer 133 shown in FIG. 2B corresponds to a movable part including the movable rod 114, the mirror 115, the support beams 117a to 117d, the movable comb tooth electrodes 121a to 121d and the hinges 118a to 118d which support the movable part in a displaceable manner while a wide width portion 136 corresponds to fixing portions such as the fixing parts 119a and 119b which are fixedly mounted on the single crystal silicon substrate 131. It is to be understood that FIG. 2B is an illustrative showing of these portions.
A wet etching is applied to the exposed insulating layer 132 shown in FIG. 2B, and the etching operation is performed until the insulating layer 132 is removed by a side etching in a region which is located below the narrow width portion 135. As a consequence, the narrow width portion 135 will be located above the single crystal silicon substrate 131 with an air gap 137 therebetween, as shown in FIG. 2C. Thus, the removal of the insulating layer 132 separates the movable part and the hinges 118a to 118d which are formed by the narrow width portions 135 from the single crystal silicon substrate 131 to be displaceable. It should be noted that portions of the single crystal silicon layer 133 which form the mirror 115 are formed with reflecting films on the lateral wall surfaces.
During the etching operation applied to the single crystal silicon layer 133, as mentioned above, it is important that the etching proceeds perpendicularly with respect to the surface of the single crystal silicon layer 133 or the sheet surface of the single crystal silicon substrate 131 to form vertical sidewall surfaces. To provide an etching process which obtains vertical sidewall surfaces, PCT Application Internationally Laid-Open WO 94/14187 (issued Jun. 23, 1994, referred to hereafter as literature 2) discloses a technique which uses plasma to continue an etching step and a polymer precipitation step in an alternate fashion.
In the optical switch constructed in the manner mentioned above, the movable part can be displaced by a flexure of the hinges 118a to 118d in a direction parallel to the sheet surface of the single crystal silicon substrate 131. Accordingly, if the hinges 118a to 118d have an increased thickness or if the hinges 118a to 118d shown in plan view of FIG. 1 have an increased width measured in a direction perpendicular to the lengthwise direction thereof, their mechanical rigidity becomes larger, requiring a drive voltage of an increased magnitude to be applied to the electrostatic comb tooth actuator in order to displace the movable part.
On the other hand, because the mechanical rigidity of the hinges 118a to 118d is proportional to the third power of the thickness thereof, the thickness of the hinges 118a to 118d has a large influence upon the dynamic response of the movable part, and hence, an extremely high accuracy is required for the thickness. In order to allow an appropriate voltage of a reduced magnitude to be used, it is necessary that the thickness of the hinges 118a to 118d be very thin on the order of 1 μm, for example, but it is not a simple matter to achieve a high accuracy in this region of sizes with the photolithography. Accordingly, a difficulty has been experienced in manufacturing an optical switch including such hinges 118a to 118d. 
This problem is not limited to an optical switch, but occurs in miniature movable devices of other kinds having a construction which supports a movable part on a substrate by hinges so as to be displaceable in a direction parallel to the sheet surface of the substrate in the similar manner as in the optical switch.